In general, in the application of an ultraviolet photodetector, while light is incident on to a p-type semiconductor layer, an undoped absorption layer and an n-type semiconductor layer of the photodetector, by means of particular bandgaps of these layers, incident photons with energy larger than the bandgap of the undoped absorption layer are absorbed by the undoped absorption layer, so that electron-hole pairs are induced within the absorption layer. Now, after a voltage is applied to the photodetector, these electron-hole pairs are driven and generate current. Therefore, an incident light can be detected by using the aforementioned theorem.
FIG. 1 is a cross-sectional view of a conventional photodetector. The photodetector constructed on the substrate 100, which is composed of sapphire, comprises the substrate 100, a nucleation layer 102, an n-type buffer layer 104, an undoped absorption layer 106, a p-type contact layer 108, a semi-transparent metal layer 110, a p-type electrode 112 and an n-type electrode 114. In the photodetector structure, the nucleation layer 102 is located on the substrate 100, the n-type buffer layer 104 is located on the nucleation layer 102, the undoped absorption layer 106 is located on a portion of the n-type buffer layer 104, the p-type contact layer 108 is located on the undoped absorption layer 106, the semi-transparent metal layer 110 is located on the p-type contact layer 108, the p-type electrode 112 is located on a portion of the semi-transparent metal layer 110 and the n-type electrode 114 is located on a portion of the n-type buffer layer 104 which is not covered by the undoped absorption layer 106. Typically, the nucleation layer 102 is composed of AlxInyGa1-x-yN, the n-type buffer layer 104 is composed of AlxInyGa1-x-yN, the undoped absorption layer 106 is composed of AlxInyGa1-x-yN, the p-type contact layer 108 is composed of p-type doped AlxInyGa1-x-yN, the semi-transparency metal layer 110 is composed of a Ni/Au stacked structure, the p-type electrode 112 is composed of a Ti/Au stacked structure and the n-type electrode 114 is composed of a Ti/Al/Ti/Au stacked structure.
When the photodetector is used to detect the incident light 116, the incident light 116 penetrates the photodetector from the p-type contact layer 108 toward the undoped absorption layer 106 and the n-type buffer layer 104. The aluminum content of the p-type contact layer 108 needs to be enhanced to increase the bandgap of the p-type contact layer 108 for the photodetector used to detect ultraviolet of high energy. However, the increase of the aluminum content in the p-type contact layer 108 reduces the conductivity of the p-type contact layer 108 and the activation efficiency of the p-type dopants, so that it is difficult to form the p-type contact layer 108 with high carrier concentration, resulting in high-resistivity p-type electrode contact 110. The semi-transparent metal layer 110 needs to be formed on a material layer with low resistivity, i.e. high hole concentration. Accordingly, while the p-type contact layer 108 has higher resistivity, the semi-transparent metal layer 110 does not have a good ohmic contact with the p-type contact layer 108, thereby degrading the performance of the photodetector. Furthermore, a portion of the electric field may be across the p-type contact layer 108 resulting from the higher resistivity of the p-type contact layer 108, thereby reducing the detecting performance of the device.
Presently, in order to solve the problem of the poor doping efficiency of the p-type semiconductor layer in the aforementioned photodetector, another photodetector is provided, such as that illustrated in FIG. 2. In the fabrication of the photodetector, a nucleation layer 202 is formed on a substrate 200, and an n-type buffer layer 204 is formed on the nucleation layer 202. Next, an undoped absorption layer 206 is formed on the n-type buffer layer 204, and a p-type contact layer 208 is formed on the undoped absorption layer 206. Then, a portion of the p-type contact layer 208 and a portion of the undoped absorption layer 206 are removed by a photolithography and etching method until a portion of the n-type buffer layer 204 is exposed. A semi-transparent metal layer 210 is formed on the p-type contact layer 208. Subsequently, a p-type electrode 212 is formed on a portion of the semi-transparent metal layer 210, and the n-type electrode 214 is formed on a portion of the exposed portion of the n-type buffer layer 204. Then, the substrate and the epitaxial layers formed thereon are inversed by a flip-chip configuration. The substrate 200 is composed of sapphire, the nucleation layer 202 is composed of AlxInyGa1-x-yN, the n-type buffer layer 204 is composed of n-type doped AlxInyGa1-x-yN, the undoped absorption layer 206 is composed of undoped AlxInyGa1-x-yN, the p-type contact layer 208 is composed of p-type doped AlaInbGa1-a-bN (x>a≧0; y≧0; b≧0), the semi-transparent metal layer 210 is composed of a Ni/Au stacked structure, and the p-type electrode 212 and the n-type electrode 214 are both composed of a Ti/Al/Ti/Au stacked structure.
When the photodetector is used to detect the incident light 216, the incident light 216 penetrates the photodetector from the substrate 200 toward the nucleation layer 202, the n-type buffer layer 204 and the undoped absorption layer 206. The n-type contact layer 204 has higher aluminum content, and it is very easy to obtain the n-type buffer layer 204 with high carrier concentration, i.e. with low resistivity, so the n-type buffer layer 204 can achieve good ohmic contact with the n-type electrode 214 formed thereon. Additionally, the incident light 216 is transmitted from the n-type buffer layer 204 to the undoped absorption layer 206, and the bandgap of the p-type contact layer 208 does not have to be higher than that of the undoped absorption layer 206, so the aluminum content of the p-type contact layer 208 does not need to be too high, and it is easy to form the p-type contact layer 208 having a low ohmic contact with the semi-transparent metal layer 210 (the semi-transparent metal layer 210 can be made of a Ni/Au stacked structure).
However, in the fabrication, after the various layers of the photodetector are formed, a flip-chip step has to be performed to inverse the substrate 200 and the layers formed thereon, so as to put the n-type buffer layer 204 above the undoped absorption layer 206, such that the incident light 216 enters the undoped absorption layer 206 from the n-type buffer layer 204 for detecting the incident light 216. Such a flip-chip step complicates the process, reducing the process yield.